This invention relates to switching the coils (windings) of an electric generator to alter the configuration of the coils of the generator in order to improve the efficiency of the generator and a system employing the generator.
The invention is of particular interest in applications where an electric generator is subjected to a wide range of operating conditions and forces such as, for example, those encountered with wave energy converters (WECs), wind turbines and, in general, any system which may be subjected to widely varying forces.
By way of example, a WEC, responsive to the motion of the waves in a body of water, may include an electric generator which is used to produce electricity in response to the motions of the waves, which can vary, in frequency and amplitude, over a very large range. Various wave energy converter (WEC) systems are known. For example, reference is made to U.S. Pat. No. 6,291,904 which issued Sep. 18, 2001, titled “Wave Energy Converter Utilizing Pressure Difference”, assigned to the assignee of the present application and the teachings of which are incorporated herein by reference.
A scheme for increasing the efficiency of a WEC is shown in U.S. Pat. No. 6,731,019 titled “Apparatus and Method for Optimizing the Power Transfer Produced by a Wave Energy Converter (WEC)” assigned to the assignee of the present application and the teachings of which are incorporated herein by reference. In this patented system, the electromechanical device is controlled and its mechanical behavior is altered to increase its energy conversion efficiency by tuning the output load. Although it is known to employ WECs to generate electric power, numerous problems exist in the design of systems for efficiently harnessing the energy contained in ocean waves because the oceans waves vary very widely in frequency and amplitude over time. Power generation, conversion and conditioning equipment is typically sized large enough to handle forces and motions resulting from extreme sea conditions. A drawback with this approach is that the same equipment is oversized and inefficient when operated in low sea-state and normal conditions. This deficiency is overcome in systems embodying the invention.
A problem encountered with widely varying voltages produced by an electric generator and its rectification may be explained with reference to FIG. 1C which shows an electric generator 100 connected to a full wave rectifying network 200. The alternating current voltage Eg produced by the generator 100 is rectified by network 200 to produce a rectified voltage (d.c.) voltage Er. Now, Er is equal to Eg minus the forward voltage drops (Vf) of two diodes (i.e., D1 and D2 for one polarity of Eg; D3 and D4 for the other polarity of Eg). For ease of discussion, the forward voltage drop (Vf) of a diode may be assumed to be in the range of 1 volt. The diode drop is a constant, fixed, drop. Consequently, if, for example, Eg is 20 volts, an efficiency loss due to the diode drops is in the range of 10%. If Eg is greater, the efficiency loss due to the fixed drops decreases. However, when Eg is less than 20 volts the efficiency loss increases, above 10%. For example, If Eg were 10 volts the 2 diode drops result in an efficiency loss of 20%. Thus for low values of Eg, the losses in the power conversion stage are very significant.
Accordingly, it is an object of this invention, to reconfigure the coils (windings) of an electric generator when its output voltage would be below a certain level in order to increase the efficiency of the conversion of the generator output. By way of example, this applies to systems in which a WEC drives an electric generator and the generator output varies considerably in response to the varying sea conditions.
Reconfiguring the coils may also be used to increase the efficiency of the driving power source, and/or the efficiency of the generator itself, by providing a better match of the source impedance to the load impedance. This problem can be explained with reference to FIG. 1C which shows an electric generator 100 connected to a full wave rectifying network 200. The generator voltage, Eg, is nearly proportional to the generator speed (angular speed in the case of rotary generators, linear speed in the case of linear generator). As speed increases, so does the generator voltage, Eg. The rectified voltage, Er, is typically required to be maintained in a narrow range in order to achieve a high-efficiency and/or simplify the design of the electric load. If Er is set to handle the generator output in heavy (“energetic”) sea conditions, then it is possible that the generator speed in low sea conditions will rarely reach the speed needed for Eg to be above Er as is required for the transfer of power from the waves, to the WEC, to the electric generator, and ultimately to the end user. This winding reconfiguration scheme is analogous to the shifting of a transmission in an automobile. It also enables the load to be better tuned to the source, for an improvement in overall efficiency.
Accordingly, it is an object of this invention, to reconfigure the coils (windings) of an electric generator when its output voltage would be below a certain level in order to increase the efficiency of the conversion of the driving power source.